What you can do

Please write a method trans that implements part of the
specification.
Since it's quite a lot to do, it's OK to only do parts. For example I
recommend to omit regexes altogether for now, and only implement literal string
patterns. The easiest approach is probably to create a hash that maps each
input pattern character to the transliteration, and walk the string character
by character by iterating over self.comb.

To test it without actually having to recompile Rakudo after each change, I
recommend to copy the test that Rakudo can parse (ie the method tests, not the
tests for tr///), and start it with

The best/most extensible/most complete/prettiest/whatever submission will
make it into the next release of Rakudo.

Update: There have been two submissions discussed
on perl6-compiler, and another one submitted as gist. If you want to work on this
challenge, please improve one of the existing submissions, not write a new one
from scratch.